Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

The following table shows stock-based compensation expense for stock options, RSUs and ESPP (in thousands):



	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development	$	1,833	$	1,445	$	3,506	$	2,841
General and administrative	1,769		1,678		3,557		3,283
Total stock-based compensation expense	$	3,602	$	3,123	$	7,063	$	6,124

Valuation Assumptions Using the Black-Scholes Option-Pricing Model

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:



	Six Months Ended June 30,
	2017	2016
Expected dividend yield	0%	0%
Expected volatility	66.7% - 67.7%	63.7% - 65.4%
Risk-free interest rate	2.0% - 2.3%	1.3% - 2.1%
Expected term	6.25 years	6.25 years

Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option and restricted stock unit (RSU) activity during the six months ended June 30, 2017:



	Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2016	3,838,060		$	18.93	7.0
Granted	1,120,950		20.38
Exercised	(155,484	)	2.36
Forfeited or expired	(182,518	)	24.13
Outstanding, June 30, 2017	4,621,008		19.63		7.2	$	17,581
As of June 30, 2017:
Exercisable	2,534,628		15.86		5.8	16,784
Vested and expected to vest	4,407,535		19.37		7.1	17,489